1. Field of the Invention
This invention relates to a toggle type mold clamping system for a vertical injection molding machine.
2. Description of the Related Art
FIG. 5 of the accompanying drawings shows the configuration of a conventional toggle type mold clamping system comprising a toggle link mechanism. The toggle link mechanism includes a pair of two links 101 connected by joints. Opposite ends of the links 101 are connected to an intermediate panel 102 and a lower panel 103, respectively. The joints are linked to a threaded shaft via screw balls, and the output shaft of a motor is connected to the threaded shaft. The threaded shaft and the screw balls function as a mechanism for driving the toggle link mechanism. A pair of molds 105 mounted on the intermediate panel 102 and an upper panel 104, respectively, are opened and clamped by operating the toggle link mechanism.
With this toggle type mold clamping system, the lower panel 103 bears the whole weight of an injection molding unit and the upper panel 104. As shown in FIG. 6(A), the drive mechanism exerts a thrust f.sub.0 in the direction to reduce the distance between the joints a, a', thereby clamping the injection molds with a force F.sub.0. On the other hand, the drive mechanism exerts a thrust f.sub.1 in the direction to increase the distance between the joints a, a', thereby opening the injection molds with a force F.sub.1 as shown in FIG. 6(B). When the foregoing weight is heavy, the thrust f.sub.1 should be larger than the thrust f.sub.0. At the final stage of the mold opening process, the links 101 are bent with an angle .alpha. as shown in FIG. 6(C). When the mold opening thrust f.sub.2 is equal to the thrust f.sub.1 in such a case, the mold opening force F.sub.2 is smaller than the mold clamping force F.sub.0. Therefore, the thrust f.sub.2' of the drive mechanism needed to open the mold or to keep the mold open should be very large as shown in FIG. 6(C). Sometimes the thrust f.sub.2' is larger than the mold clamping thrust f.sub.0. A motor having a large torque is required to turn the threaded shaft and to obtain the thrust f.sub.2' for generating the force F.sub.2.
To overcome this problem, the larger the minimum bending angle .alpha. of the toggle links, the smaller the mold opening stroke. To ensure a sufficient mold opening stroke, the links should be longer, which will extensively enlarge the mold clamping system.